The present invention relates to an automatic correction device in which the deviation of a vehicle from a running lane is automatically monitored and, when the deviation occurs, a brake pressure applied to at least one of the left and right wheel brakes is automatically increased, thereby automatically adjusting a distribution between pressures applied to left and right wheel brakes and returning the vehicle to the running lane. Hereafter, an adjustment of brake pressure distribution between left and right wheel brakes, i.e., a change of vehicle's travelling direction by a differential braking, is referred to as "brake steering".
The above automatic correction of travelling direction is effective for temporarily securing the safety of the running vehicle, when a driver's watchfulness ahead of the running vehicle deteriorates, e.g., when the driver looks aside, falls asleep or is in a semi-comatose state before or after sleeping.
In "A Warning and Intervention System to Prevent Road-Departure Accidents" recited in Vehicle System Dynamics Supplement 25 (1996), pp 383-396, a vehicle travelling direction is automatically adjusted in the direction along which the vehicle moves by a feedback control in which a front view ahead of the vehicle is photographed by a television camera and a running lane is detected by image processing, vehicle behavior is inferred from information detected by another sensor mounted on the vehicle, and when an unintended deviation from the running lane occurs, the amount of deviation determines the amount of control with respect to wheel brake pressure distribution.
In "Correlation between Snaking of Vehicle and Awakening Degree" recited in Japan Automobile Technology Association's Scientific Lecture Preprints 941, pp. 25-28 published in May 1994, there is suggested a technique in which the front view ahead of the vehicle is photographed by a television camera, a white line partitioning a running lane is detected by image processing, and a lateral shift amount of the vehicle is computed, thereby detecting a snaking state of vehicle.
Detection of a running lane and detections of lane width, curve, a preceding vehicle or the like have been already suggested by the present applicant (for example, Japanese Patent Unexamined Publication No. 6-213660). Further, there has been suggested a technique in which a television camera for photographing a front view ahead of the vehicle is turned to follow a lane curve, thereby tracing a forward running lane (for example, Japanese Patent Unexamined Publication No. 9-96507). Furthermore, there has been suggested a distribution control technique for wheel brakes (for example, Japanese Patent Unexamined Publication No. 8-207737). By combining these techniques, the aforementioned automatic correction of travelling direction can be realized.
It is inferred that in the feedback control in which the amount of deviation determines the amount of control with respect to wheel brake pressure distribution, when the deviation amount is large, an increase in pressure for a wheel brake is high and when the former is small, the latter is low, so that the effect of correcting the deviation amount is high, but when the deviation amount is large a change in direction of the vehicle is great, so that depending on road conditions such an unstable behavior is liable to occur that an increase in wheel brake pressure becomes too high so the wheels are locked or that a change in direction of vehicle is so sharp the vehicle spins. Steering by a driver during running of a vehicle is performed generally in response to the car speed, a radius of curvature and the friction coefficient of the road surface. It is preferable that brake steering for the aforementioned automatic correction of travelling direction responds smoothly to a car speed, a radius of curve and a friction coefficient of road surface (these are combined and referred to as "running conditions"), and it is considered that brake steering not reflecting a driver's will should lay emphasis on a stable and gentle correction of direction rather than a rapid correction of the deviation amount. It is inferred that by introducing such parameters as a car speed, a radius of a curve, a yaw rate and the like to the aforementioned feedback control and revising the amount of operation in response to the above parameters, there is obtained an improvement in consistency and smoothness with respect to the running conditions. However, the characteristic that since the control amount is the deviation amount, if the deviation amount is large an increase in wheel brake pressure is high and a change in direction is large will be maintained because that characteristic is an object inherent in the aforementioned feedback control.
A driver in another vehicle is uneasy if a preceding vehicle, or a vehicle running in the opposite direction, rapidly changes its direction. In the event that another vehicle deviates from the lane, it is easier to cope with that vehicle when it returns slowly but stably and gradually to the lane judging from its running state than when it exhibits such a behavior that it returns rapidly to the lane.
It cannot be said that reliability of detecting a vehicle in the running lane is sufficient. Further, it is comparatively frequent that a deviation from the running lane or a change of the running lane is performed by a normal judgment of the driver in order to avoid another vehicle or an obstacle. Alternatively, the running lane is frequently changed as a result of the driver's intent. However, in these cases, a consistency between the driver's will and the brake steering is low. Therefore, it is preferable that a brake steering amount is made as small as possible in order to prevent a hindrance in driving the vehicle from occurring. To the contrary, when the driver's power of attention deteriorates (e.g., when the driver looks aside or falls asleep), it is preferable that the brake steering is strongly applied in the event that the vehicle deviates from the lane. However, at present it is difficult to realize these problems simultaneously by a feedback control.
Further, in a usual road in which there are many parked or stopped vehicles and many telephone or electric poles, the radius of curvature is small or across which many people and vehicles traverse, an error in detecting the running lane is liable to occur. Further, even if the running lane is accurately detected, a change of running lanes and a deviation from a lane is frequently performed. Under such situations, in view of the driver's recognition and intention, it is highly possible that the aforementioned automatic correction of travelling direction becomes a malfunction and becomes an erroneous interference to the driver. When a vehicle runs on a large road, e.g., a freeway (road exclusively for vehicles), on which it can keep a comparatively high speed with little possibility of sharp steering for avoiding another vehicle or the like and whose smallest radius of curve is comparatively large, driving at a substantially constant speed with little steering continues for a long time. For example, there is also a situation where the driver selects an automatic cruising which conducts an intra-vehicle control or a fixed speed control. Under such running conditions, since the incentive to stay awake is small, it is liable to make the driver sleepy. The aforementioned automatic correction of travelling direction is effective for supplementing the driver's carelessness under such running conditions.